It is well known to mount a blue LED die in a conical reflective cup and then fill the cup with a drop of a viscous phosphor mixture. The mixture is then cured. If the phosphor is a YAG phosphor, white light will be created by the combination of the yellow-green YAG emission and blue light leaking through the cured phosphor mixture.
A common phosphor mixture comprises phosphor powder in a transparent binder, such as silicone or epoxy. Such binders have very poor thermal conductivity (TC), such as 0.1-0.2 W/mK, and make up the bulk of the phosphor mixture. Accordingly, the overall TC of the hardened phosphor mixture is very low.
Due to the low TC, heat is not conducted from the LED die through the phosphor mixture and to the cup walls and base. Accordingly, the LED operates at a relatively high temperature. If the LED operates at a high temperature, its light output becomes lower and its color shifts over its lifetime. Therefore, the low TC phosphor mixture limits such reflector cup applications to low and medium current/heat LEDs.
What is needed is a technique that allows a high brightness/high current/high heat LED to be encapsulated by a phosphor mixture in a reflective cup.
In addition to the problem addressed above, the phosphor mixture must have a low viscosity in order for it to be used with a small-diameter dispensing nozzle under practical injection pressures and to fill in the voids around the LED die. A typical phosphor mixture may produce uneven phosphor density in the deposited mixture due to the rapid slurry flow and the low viscosity of the mixture. Additionally, the phosphor mixture viscosity permits phosphor particles in the dispensed volume to settle to the bottom of the cup before the phosphor mixture is cured. Due to this phosphor settling inside the cup, it is difficult to maintain the targeted color of the package. Further, it is difficult to repeatedly dispense identical amounts of the viscous phosphor mixture in the cups, which causes non-uniformity of color from LED module to LED module.
What is needed is a technique to improve the color consistency from LED module to LED module and to improve the color uniformity of the package.